Prostratin is a nontumor-promoting 12-deoxyphorbol ester that has been shown to inhibit HIV-induced cell death and viral replication in vitro. The antiviral activity of this compound was discovered as a result of ethnobotanical studies on the island of Savai'i, Samoa, where traditional healers use the bark of Homalanthus nutans (G. Forst.) Guill. (Euphorbiaceae), a small rain-forest tree called “mamala”, to treat hepatitis. Low concentrations of prostratin, from 0.1 to >25 M, have been found to protect T-lymphoblastoid CEM-SS and C-8166 cells from the lethal effects of HIV-1 and inhibit viral replication in these cell lines. The compound also demonstrated cytoprotective activity in the human monocytic cell line U937 and in freshly isolated human monocyte/macrophage cultures. Prostratin was found to bind to and activate protein kinase C in vitro in CEM-SS cells. Unlike other known phorbol esters, however, the compound has proved not to be a tumor promoter and has actually been shown to be a potent antitumor promoter.
In latently infected CD4+ T cells, prostratin induces HIV gene expression. NF-κB and PKC (α and θ) activation are the key events triggered by prostratin. Although other phorbol esters such as PMA (phorbol 12-myristate-13-acetate) are also shown to activate latent HIV, prostratin differs markedly from these and offers distinct therapeutic value because it does not exhibit the tumor-promoting activity of these other agents. Therefore, prostratin is a promising therapeutic lead as an adjuvant to be used in HAART (highly active antiretroviral therapy).
In an in vitro study, prostratin was shown to protect T-lymphoblastoid CEM-SS and C-8166 cell lines. At a prostratin concentration of approximately 1 μM, cell viability was restored to the level of uninfected controls, and no sign of cytotoxicity was observed up to about 25 μM. The mode of action is unclear, but the Ki of prostratin for PKC is 12 nM, suggesting the involvement of PKC in the process. (Gulakowski, R. J.; McMahon, J. B.; Buckheit, R. W., Jr.; Gustafson, K. R.; Boyd, M. R. Antireplicative and anti cytopathic activities of prostratin, a non-tumor-promoting phorbol ester, against human immunodeficiency virus (HIV). Antiviral Res 1997, 33, 87-97.)
Prostratin inhibits HIV invasion into healthy cells by downregulating the expression of HIV receptors on cell surfaces
In CEM-SS and MT-4 cell lines, CD4 receptors were significantly reduced on cell surfaces, and mRNA array assay confirmed that CD4 gene expression along with other HIV-1 receptors (CXCR4 and CCR5) were downregulated in THP-1 cells. Staurosporine, a PKC inhibitor, was shown to reverse the CD4 downregulation by prostratin, implying the involvement of PKC activation in the process. In addition, prostratin stimulates the internalization and subsequent degradation of CD4 and CXCR4 receptors in CEM cells. PKC translocation studies on this cell line showed PKCβ and PKCδ remained in the cytosol, whereas PKC and c were effectively translocated to the plasma membrane (Gustafson, K. R; Cardellina, J. H., 2nd; McMahon, J. B.; Gulakowski, R. J.; Ishitoya, J.; Szallasi, Z.; Lewin, N. E.; Blumberg, P. M.; Weislow, O, S.; Beutler, J. A.; et al., A non promoting phorbol from the Samoan medicinal plant Homalanthus nutans inhibits cell killing by HIV-1. J. Med. Chem. 1992, 35, 1978-1986. (b) Wang, Y. B.; Huang, R.; Wang, H. B.; Jin, H. Z.; Lou, L. G.; Qin, G. W. Diterpenoids from the roots of Euphorbia fischeriana. J. Nat. Prod. 2006, 69, 967-970.)
In a more recent study, DPP (12-deoxyphorbol 13-phenylacetate), another non-tumor promoting phorbol ester, was reported to be 20-30 fold more potent than prostratin in activating latent HIV-1. DPP also downregulates CD4 and CXCR4 receptors at nanomolar concentrations. (Bocklandt, S.; Blumberg, P. M.; Hamer, D. H. Activation of latent HIV-1 expression by the potent anti-tumor promoter 12-deoxyphorbol 13-phenylacetate. Antiviral Res. 2003, 59, 89-98.)